Image forming apparatuses (below called liquid ejecting image forming apparatuses) are known which eject liquid droplets onto sheet material such as a sheet of paper to form an image. The liquid ejecting image forming apparatuses may generally be divided into a serial-type image forming apparatus and a line-head type image forming apparatus. In the serial-type image forming apparatus, a recording head moves in both main scanning directions perpendicular to a direction of sheet conveying while the sheet conveying is repeated to form an image over the sheet of paper. In the line head-type image forming apparatus with nozzles being aligned in a length which is almost the same length as a maximum width of the sheet of paper, when a timing arrives at which the sheet of paper is conveyed and the liquid droplets are ejected, nozzles within the line head eject the liquid droplets to form the image.
However, it is known that, in the serial-type image forming apparatus, when one ruled line is printed in both directions of an outward path and a return path, an offset of the ruled line is likely to occur between the outward path and the return path. Moreover, it is known that, in the line head-type image forming apparatus, parallel lines are likely to appear in the sheet-conveying direction when there is a nozzle whose position of impacting is constantly offset due to a mounting error, a finishing accuracy of the nozzle, etc.
Therefore, in the liquid-ejecting image forming apparatus, it is often the case that a test pattern for self-adjustment to adjust the position of impacting the liquid droplets is printed on the sheet material, the test pattern is optically read, and an ejection timing is adjusted based on the read results (see Patent document 1, for example.)
Patent document 1 discloses an image forming apparatus which includes a pattern forming unit that forms, on a water-repellent member, a reference pattern including multiple independent liquid droplets and a pattern to be measured that includes multiple independent liquid droplets ejected under an ejection condition different from the reference pattern such that they are aligned in a scanning direction of a recording head; a reading unit including a light emitting unit which irradiates a light onto the respective patterns and a light receiving unit which receives a regular reflected light from the respective patterns; and a correction unit which measures a distance between the respective patterns based on read results of the reading unit for correcting of a liquid droplet ejection timing of the recording head based on the measurement results.